1. Critical Systems Specification
IS301 – Software Engineering
Lecture # 9 –
M. E. Kabay, PhD, CISSP
Assoc. Prof. Information Assurance Division of Business & Management, Norwich University V:
M. E. Kabay, PhD, CISSP
Copyright © 2004 M. E. Kabay All rights reserved.

2. Today we will use 18 of Prof. Sommerville’s slides in class
Topics covered
Risk-driven specification
Safety specification
Security specification
Software reliability specification
Today we will use 18 of Prof. Sommerville’s slides in class

3. Dependability requirements
Functional requirements to define error checking and recovery facilities and protection against system failures.
Non-functional requirements defining the required reliability and availability of the system.
Excluding requirements that define states and conditions that must not arise.

4. Risk-driven specification

5. Levels of risk Unacceptable region Risk cannot be tolerated
Risk tolerated only if risk reduction is impractical or grossly expensive
Acceptable
region
ALARP region
Neglible risk

6. Risk assessment - insulin pump

7. Fault-tree analysis A deductive top-down technique.
Put the risk or hazard at the root of the tree and identify the system states that could lead to that hazard.
Where appropriate, link these with ‘and’ or ‘or’ conditions.
A goal should be to minimize the number of single causes of system failure.

8. Insulin pump fault tree

9. Control system safety requirements

10. Copyright © 2004 M. E. Kabay. All rights reserved.
The safety life-cycle
Copyright © 2004 M. E. Kabay All rights reserved.

11. The security specification process

12. Types of security requirement
Identification requirements.
Authentication requirements.
Authorization requirements.
Immunity requirements.
Integrity requirements.
Intrusion detection requirements.
Non-repudiation requirements.
Privacy requirements.
Security auditing requirements.
System maintenance security requirements.

13. Copyright © 2004 M. E. Kabay. All rights reserved.
Reliability metrics
Reliability metrics are units of measurement of system reliability.
System reliability is measured by counting the number of operational failures and, where appropriate, relating these to the demands made on the system and the time that the system has been operational.
A long-term measurement program is required to assess the reliability of critical systems.
Copyright © 2004 M. E. Kabay All rights reserved.

14. Reliability metrics

15. Probability of failure on demand
This is the probability that the system will fail when a service request is made. Useful when demands for service are intermittent and relatively infrequent.
Appropriate for protection systems where services are demanded occasionally and where there are serious consequence if the service is not delivered.
Relevant for many safety-critical systems with exception management components
Emergency shutdown system in a chemical plant.

16. Rate of fault occurrence (ROCOF)
Reflects the rate of occurrence of failure in the system.
ROCOF of means 2 failures are likely in each 1000 operational time units e.g. 2 failures per 1000 hours of operation.
Relevant for operating systems, transaction processing systems where the system has to process a large number of similar requests that are relatively frequent
Credit card processing system, airline booking system.

17. Mean time to failure
Measure of the time between observed failures of the system. Is the reciprocal of ROCOF for stable systems.
MTTF of 500 means that the mean time between failures is 500 time units.
Relevant for systems with long transactions i.e. where system processing takes a long time. MTTF should be longer than transaction length
Computer-aided design systems where a designer will work on a design for several hours, word processor systems.

18. Availability
Measure of the fraction of the time that the system is available for use.
Takes repair and restart time into account
Availability of means software is available for 998 out of 1000 time units.
Relevant for non-stop, continuously running systems
telephone switching systems, railway signaling systems.

19. Homework Required By Mon 27 Sep 2004
Write an essay of words discussing question 9.1 for 15 points
Answer questions 9.2, 9.3 & 9.6 for 10 points each
For 20 points, answer question 9.9 in detail. Note the phrase “Giving reasons…”
Optional
By Monday 4 Oct 2004
Q. 9.8 for 12 points with reasons
Q and/or 2 points each

20. QUIZ #1 IN CLASS WED 22 SEP 2004 Chapters 1 - 5
REMINDER
QUIZ #1 IN CLASS
WED 22 SEP 2004
Chapters 1 - 5

21. Copyright © 2004 M. E. Kabay. All rights reserved.
DISCUSSION
Copyright © 2004 M. E. Kabay All rights reserved.